The subject matter of the present application is related to that of U.S. Pat. No. 4,804,132, naming Louis DiFrancesco as the sole inventor, which issued on Feb. 14, 1989.
1. Field of the Invention
The present invention relates generally to the mechanical, electrical, and thermal connection of metal surfaces and, more particularly, to methods for forming permanent and temporary junctions between metal surfaces having metallized particles therebetween.
Numerous techniques exist for joining metal surfaces together. Typical examples include the use of fasteners, adhesives, solders, and the like, as well as various welding and ultrasonic bonding methods. Of particular interest to the present invention are compression bonding techniques where opposed metal surfaces are brought in contact and bonded by applying sufficient pressure, and optionally heat, to cause metallic or electron bonding between adjacent metal molecules. When heat is applied in addition to pressure, such bonding methods are referred to as thermocompression bonding.
The amount of pressure, and optionally heat, required to achieve compression bonding will vary depending on a variety of circumstances. For example, soft metals having a relatively low melting point can usually be bonded under less rigorous conditions than harder metals having higher melting points. Similarly, permanent bonds having a strength generally equal to that of the bulk metal being joined generally require more rigorous bonding conditions than do temporary bonds having a strength much less than that of the bulk material. Of course, the longer exposure times can in some cases provide bonds having strengths equal to that achieved under more rigorous conditions for lesser times. Finally, the surface area of the bond has a substantial impact on the amount of energy in the form of temperature and pressure which must be applied. Compression bonding techniques which are suitable for very small areas are often difficult or impossible to achieve with very large areas of metal to be joined.
Under many circumstances, it would be desirable to employ lower pressures and/or to eliminate the need to use heat while being able to achieve the same quality of bond achieved using more rigorous conditions. In particular, it would often be desirable to utilize reduced pressure and/or temperature without the need to extend the time required for effecting bonding. Such compression bonding techniques which utilize reduced pressure and temperature would find particular use in the electronics industry in various interconnection applications, such as die attach, peripheral pad bonding, wire bonding, tape bonding, and other electrical conductor bonding situations. In such applications, it would also be desirable to be able to form temporary bonds, allowing automatic testing of associated circuitry, prior to forming permanent bonds.
Under other circumstances, it would be desirable to be able to make compression bonds over relatively large surface areas without excessive pressure or heat requirements. For example, it may be desirable to join metallic structural members over large areas to assure the strength and integrity of the resulting joint. Because of the large size and mass of such structural members, however, the heat requirements which have heretofore been necessary to achieve thermocompression bonding or welding would often result in deforming or altering material properties of the structural element. Additionally, it is often very difficult to apply pressure to large members in the manner required for forming compression bonds, and it is often difficult or impossible to employ compression bonding techniques in the field.
As described in more detail hereinbelow, the methods of the present invention are broadly applicable in many diverse metal bonding applications and enjoy a number of specific advantages which are enumerated.
2. Description of the Background Art
U.S. Pat. No. 4,814,040, describes a method for connecting electrical contact pads employing an adhesive layer which contains a plurality of metallic particles. The particles penetrate into the contact pads to enhance electrical contact, but no diffusion bonding between the pads or between the particles and the pads is disclosed. U.S. Pat. No. 3,818,415, discloses a method for electrically coupling a terminal to a conductor coated with an insulating layer. Fine particles of grit are adhered to the conductor and coated with a soft metal. The grit scores the insulating layer as the terminal is moved thereacross, and electrical conduction is established by the soft metal. No bonding between the metal and conductor is disclosed. Japanese patent 63/264283, describes diffusion bonding of nickel alloy surfaces having a nickel alloy powder therebetween. U.S.S.R. patents 1333511 and 893469, describe diffusion welding of metal surfaces having a metallic powder therebetween. Japanese patent 63/049382, describes diffusion bonding between metals using an insert layer having low melting point and high melting point particulates therebetween.